A turbocharger consists of two turbine wheels mounted on a single shaft. Exhaust gases enter one wheel, driving it. Simultaneously with the rotation of the first wheel, the second one rotates, forcing atmospheric air into the combustion chambers (see illustration 16.0).
With such operation of the turbocharger, with an increase in its speed, excess air will be forced into the combustion chambers and the boost pressure will become such that the engine will not withstand it (see illustration 16.0a).
To avoid this situation, TD engines are equipped with a boost pressure reducing valve. This valve opens, allowing only part of the exhaust gas to reach the compressor turbine wheel, thus maintaining the boost pressure within 0.83 bar.
The TDI engine is also equipped with a regulator. However, in this case, the regulator functions differently than in TD engines and limits the boost air pressure. This is done using a solenoid valve located at the rear of the engine compartment on the right side. This valve opens the pressure reducing valve via a rod from the diaphragm valve, reducing the boost air pressure. The solenoid valve is controlled by the fuel system control unit according to information from the intake manifold pressure sensor. The intake manifold pressure sensor is connected to the intercooler via a thin hose (charge air cooler). Thus, the boost pressure regulator is able to compensate for the decrease in air density in areas with high altitudes.
Boost Air Pressure Regulator Solenoid Valve - Check
1. Make sure that the hoses attached to the valve are not mixed up.
2. Measure the internal resistance of the valve. To do this, disconnect the valve plug.
3. Connect a sensitive ohmmeter to both contacts of the plug. It should show 24-28 ohms. Otherwise, the boost pressure limiting solenoid valve is faulty and must be replaced.
4. Check the supply of power to the solenoid valve. To do this, connect the voltmeter to terminal 2 of the connecting plug and to the "ground" (see illustration).
5. Turn on the ignition. The voltmeter should show the battery voltage. Otherwise, the power supply wire is broken.
The procedure for checking the intake manifold pressure sensor is indicated in the relevant chapter.
Boost Air Pressure Limiting Diaphragm Valve - Check
6. Warm up the engine to operating temperature.
7. Check the plug of the boost pressure limiting solenoid valve.
8. Start the engine and increase its speed to maximum. The pressure reducing valve drive rod from the diaphragm valve should shift (see illustration).
Note: Shutting down the engine immediately after a long drive at full load is harmful to the turbocharger. This treatment of the turbocharger can lead to "coking" of the supercharger. The shaft bearing next to the turbine wheel heats up to such an extent due to the sudden accumulation of hot air that all the grease in the bearing burns and sticks to the rubbing surfaces. If the engine is constantly shut down in this way, the turbocharger will not last long. Therefore, after a long drive in the "Full Load" mode, it is recommended not to shut off the engine, but to let it idle for a while.
